Large, lightweight mirrors are used in a variety of space applications, as well as in many other fields. In space activities, for example, it is important that the mirrors be as light in weight as possible, so as to lessen the payload that must be carried into space. Also, in adaptive optics fields wherein deformable mirrors are utilized, forces must be applied to thin sections of the mirror to deform it, and the less the weight of the mirror, the less the force that must be applied, and the smaller the force generators (actuators) and flexures that are required.
Nothwithstanding the desire for the mirror to be as light in weight as possible, it must, at the same time, have sufficient rigidity and stability so that it can be efficiently controlled and/or otherwise handled. It is taught in the prior art, for example, that the weight of a glass mirror can be reduced by simply forming a large number of cavities or pockets in the back face of the mirror support. This can be done by physically machining holes in the back surface of the mirror to create a honeycomb structure or the like, or alternatively, a very thin facesheet of ULE, fused silica, etc., can be used together with appropriate stiffening elements which are attached to it by cementing or by fusing to provide the needed support.
In the embodiment wherein the mirror is reduced in weight by machining, it is obvious that the time and expense involved in completing the mirror will be substantial. The use of epoxy to bond a facesheet to stiffening elements is also not a particularly desirable approach because it has been found that substantial cracking and chipping tends to appear in the glass at even carefully controlled fillets of epoxy, at the very low temperatures encountered in space. Also, tests have shown evidence of initial atmospheric moisture causing separation of epoxy bonded glass to glass joints during thermal cycling to very low temperatures. Fusing is also done by firing to the softening point of the glass, and therefore carries considerable risk of distortion and spoilage.
Thus, there is a real need for an improved method of making lightweight glass mirrors that will avoid the disadvantages described above.